Microphone converter



Nov. 12, 1957 H w 2,812,826

MICROPHONE CONVERTER Filed March 51, 1954 2 Sheets-Sheet l [I r' i 1 I30 7 12 4 4 4 2 XIII f a v INVENTOR.

Mimi/Z2111 BY 1! TTORNEY Nov. 12, 1957 M. E. HAWLEY 2,812,826

MICROPHONE CONVERTER Filed March 31, 1954 2 Sheets-Sheet 2 IN VE N TOR.

I BY

d TTORNE 1' United States Patent 2,812,826 MICROPHONE CONVERTER Moises E. Hawley, Collingswood, N. J., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application March 31, 1954, Serial No. 420,072 9 Claims. (Cl. 181-34) This invention relates to higher order gradient microphones, and more particularly to adapters for converting a first order pressure gradient microphone to a higher order pressure gradient microphone.

A pressure gradient microphone is one at which the voltage output is proportional to a derivative of the sound pressure with respect to distance. The order of the gradient operation is equal to the order of the derivative, i. e. the output of the second order pressure gradient microphone is proportional to the second derivative. These microphones are valuable for noise discrimination properties, and, in general the higher the order of the gradient operation the greater the noise discrimination.

In practice, first order noise cancelling microphones are usually constructed by opening the two sides of the diaphragm to the ambient. Until recently, higher order microphones were built out of combinations of pressure microphones or first order microphones. The sensitivity, frequency response characteristics, and phase of all the units involved in such a higher order microphone must be very nearly equal, and the higher the order, the closer the tolerance of said operation. It is apparent, therefore, that it would be desirable to have a sound translating structure or microphone of the higher order type with as few elements as possible.

It is much easier to balance the frequency response and sensitivity of a first order microphone than it is to balance the frequency response and sensitivity of higher order microphones. It would, therefore, be desirable to have a relatively easy means of converting balanced first order microphones to microphones of the higher type.

In accordance with the present invention, means are provided to attach to a first order pressure microphone to yield higher order pressure gradient operation. A container or casing having top and bottom members is divided into geometrically equal chambers by a rigid wall erected along the diameter of the container. A hole in the bottom member is adapted to acoustically connect each chamber to one side of a vibratile element of a first order pressure gradient microphone. The top member has apertures to admit the sound pressure to the proper chambers. The holes must be of predetermined sizes and properly positioned to provide the correct acoustic impedance for third order operation.

It is an object of this invention to provide a meaz. for converting a first order microphone to a higher order microphone with a minimum number of parts.

It is a further object of this invention to provide means for converting a first order microphoneto a higher order microphone by means of adapters which ar relatively cheap and easy to manufacture.

Other objects and advantages of the present inv will become apparent and suggest themselves to skilled in the art to which the invention is directed from a reading of the following specification in connection with the accompanying drawing in which:

Figure 1 is an exploded view of an adapter embodying the present invention;

Figure 2 is a top view of the assembled adapter illustrated in Figure 1;

Figure 3 is a cross-sectional view of the adapter shown in Figure 2, the section being taken along line 33;

Figure 4 shows the adapter illustrated in Figure 1 associated with a first order gradient microphone;

Figure 5 shows a device embodying the invention in another fo rm;

Figure 6 is a cross-sectional view of the device shown in Figure 5, the section being taken along line 6-6;

Figure 7 is a cap adapted to fit the device shown in Figure 5;

Figure 8 is a cross-sectional view of the cap shown in Figure 7, the section being taken along the line 8-8;

Figure 9 illustrates another cap embodying the invention and adapted to fit the device shown in Figure 5;

Figure 10 is a cross-sectional view of the cap shown in Figure 9, the section being taken along line 10-10;

Figure 11 is still another cap embodying the invention and adapted to fit the device shown in Figure 5; and

Figure 12 is a cross-sectional view of the cap shown in Figure 11, the section being taken along line 12-12.

theory leading to the development of the present invention. In accordance with this theory, it can be shown that fewer elements will sutfice for pressure gradient responsive microphones than those heretofore provided. Suppose, for example, that one Wishes to build a second order microphone. If four pressure microphones are used, let their respective pressures be designated as P1, P2, P3, P4. Their method of combination for second order operation would be as follows:

By utilizing the second pressure P2 twice, one can obtain, for the first pressure difference (P1P2) and for the second pressure difference (Pz-Ps). These may be combined and expressed as follows to obtain second order operation:

(P1P2)(P2P3) Expression 1 may be simplified and written as follows:

P1 2P2 +Ps Similarly for a third order microphone, instead of using eight pressure microphones only five are necessary since one can utilize pressures P2, P and P4 twice. Thus, for the first pressure difference one obtains (P1 Pz), for the second pressure difference one has (P2P3), for the third pressure difference one has (P3P4) and for the fourth pressure difference (P4P5). These may be combined and expressed as follows to obtain third order operation:

[(P1P2)(P2P3)][(P3-P4)(P4P5)] (4) Expression 4 may be simplified and written as follows:

P1 -2P2 I 2P4 P5 (5) It will be noted that the third pressure P3 cancels out. Therefore, it is not necessary to utilize the microphone providing pressure P3 at all. Thus, for any order of sure may be additive in one place and subtractive in another. This is the case when the pressure is used as the microphone of the same order gradient operation. For example, reference is made to Expression 4 above wherein P3 is the last pressure of the other second order, pressure gradient responsive microphone. So far as the resultant pressure is concerned, pressures which are added in one place and-subtracted in another may be, omitted entirely providing the proper spatial distributionof the other pressures is not changed.

One method of attaining a third order'pr'essure gradient microphone response is to conduct thesound from each pressure pick-up'point to the diaphragm cavity through separate channels and to mix them in the cavity. An other is to combine the sound from the pick-up points and lead the combinations to the diaphragm cavities. The second method is embodied in the present invention and permits relatively simple low-attenuation conduction in the system over the first method, although the first methodmay have other advantages. Another. advantage of this invention is that the equivalent impedances of the acoustical paths of the first order gradientmicrophone may be determined before proceeding to higher order operation.

On the basis of this theory, a third order, pressure gradient responsive microphone may be constructed which uses but a single pressure-sensitive element. Some of the embodiments constructed and operated, in which adapters were used to convert a first order pressure gradient microphone to a third order type, are shown in this invention. Of course, the principles involved in this invention may be employed to convert a first order gradient microphone to a second order gradient microphone. 7 V

Referring particularly to Figures 1, 2, and 3, a cylindrical cavity or casing 18 contains a wall 12 which (ll-f vides the cavity into two geometrically similar compartments 14 and 16. A cap or top member 18 has four apertures 29, 22, 24 and 26. A bottom member 28 has two holes or apertures 30 and 32. The four apertures on the cap member are disposed in a relatively straight line. The distance between the apertures 20 and 22 is equal to the distance between the apertures 24 and 26. Each of these distances is equal to one-half of the distance between the apertures 22 and 24. These apertures serve as a series of sound pressure pick-up points.

The wall dividing the cavity into two compartments describes a serpentine path and is so constructed that the apertures 20 and 24 lead to one compartment 14 and the apertures 22 and 26 lead to the other compartment 16. The apertures 30 and 32 of the bottom member lead to the compartments 14 and 16, respectively.

The apertures 20 and 26 located at the extreme positions on cap member are dimensioned to provide equal acoustic impedance. Likewise, the apertures 22 and 24 located in the mean positions are dimensioned to provide equal acoustic impedance. The apertures 20 and 26 in the extreme positions are made substantially one-half the size of the apertures 22 and 24 in the mean positions, thereby making the acoustic impedances of the extreme apertures 20 and 26 substantially twice that of the mean apertures 22 and 24. V t

A similar arrangement may be used to provide an adapter to convert a first order gradient microphone to a second order gradient microphone. In this case only three apertures would be required to connect the ambient to the acoustic chamber. In such a device, the central aperture would have one-half the acoustic impedance of either of the extreme apertures, with the central aperture leading to one chamber and the extreme apertures leading the the other chamber. In view of this, it will be noted that all of the embodiments disclosed herein may be modified and used to provide an adapter to convert a first order microphone to a third order microphone.

Referring particularly to Figure 4, the converter illustrated in Figures 1, 2 and 3 is shown attached to a first order gradient microphone 34. Acoustically identical tubes 36 and 38 lead from the two holes 30 and 32 in thebottom member of the converter to the entrance ports of the first order gradient microphone. Details of the equal chambers 48 and 50 first order gradient microphone are conventional and, therefore, are not shown for purposes of simplification. Reference may be had to the text, Elements of Acousti cal Engineering, by H. F. Olson, Section 8.3 for a further discussion of first order gradient microphones. It is understood, however, that each of the tubes 36 and 38 leads to a difierent side of a-diaphragm or vibratile ele ment of the first order gradient microphone. The output from the first order gradient microphone is taken from terminals 40 and 42 by means of a conductor 44. Referring to Figures 5 and 6, a rigid wall 52 divides a cylindrical device or container 46 into two geometrically erected along a diametral plane of the container.

An aperture 54 is adapted to lead from the chamber 48 to an entrance tube of a first order gradient microphone, such as shown in Figure 4. A second aperture 56 is adapted to lead from the chamber 50 to another entrance tube of this first order pressure gradient microphone. As was the case in the embodiment shown in Figure 1, the two chambers are adapted to be acoustically coupled to opposite sides of a diaphragm of'a first order gradient microphone. It is noted that the wall 52 extends slightly higher than the side Walls of the container to permit easy attachment of a cap member, to be described.

Referring particularly to Figures 7 and 8, a cap or cover 58 having substantially the same circumference as the container comprises a circular plate having a plurality of apertures 60, 62, 64 and 66 are staggered in series spaced apart relationship to admit the sound pressure to alternate chambers of the cylindrical container. The outer apertures 60 and 66 have approximately one-half the area of the inner apertures 62 and 64. The acoustic impedance of each of the outer apertures 60 and 66 is substantially twice that of each of the inner apertures 62 and 64. The cap is provided with a groove or indentation 67 cut out to permit insertion of the wall 52 of the container. The cap may then be glued or otherwise connected to the container by any suitable means.

The apertures in the cap are disposed to acoustically conduct the sound pressure entering the apertures 60 and 64 to one chamber 50, and the pressure entering the apertures 62 and 66 to the other chamber 52.

Referring particularly to Figures 11 and 12, there is shown another cap ,adapted to be connected to the container 46 in accordance with the present invention. A cap 82 has four apertures 84, 86, 88 and 90 aligned in substantially a straight line and drilled at an angle less than 901 to the surface of the cap in order to conduct sound from the ambient to the proper chambers. It is seen that the apertures 84 and 88 lead to one chamber while the apertures 86 and 90 lead to the other chamber. The sizes of the apertures 86 and 88 are proportioned so as to provide one-half the acoustic impedance of the apertures 84 and 90. The outer apertures have approximately one-half the area of the inner apertures. Again a groove or indentation 92 is provided to permit insertion of the wall member 52, when the cap is suitably attached to the container.

Referring in particular to Figures 9 and 10, there is shown another cap adapted to be connected to the container 46 in accordance with the invention. The cap 68 comprises six apertures 70, 72, 74, 76, 78 and 80 of equal diameter.

A pair of apertures 72 and 74 are disposed in parallel relationship with a second pair of apertures 76 and 78. The two pairs of apertures occupy the mean positions. Apertures 70 and 80 occupy the extreme positions. The impedance of each of the four apertures in the mean positions is equal to the acoustic impedance of each of the apertures in the extreme positions. Two pairs of apertures are disposed in parallel position at the mean positions so that each pair provides one-half the acoustic impedance tures are of each of the outer apertures. The aperdrilled through the cap at an angle of less amasse than 90 degrees to permit sound pressures from the ambient to be acoustically coupled to the proper chambers. It is seen that the ap rture 70 and the pair of apertures 76 and 78 lead to one chamber While the aperture 80 and the pair of apertures 72 and 74 lead to the other chamber. Drilling all the apertures to the same dimensions facilitates the manufacture of such cap members and makes it easier to provide the exact acoustic impedances desired. A groove or indentation 81 is provided to permit insertion of the wall member 52. The cap may be mechanically connected to the container by any suitable means.

Although it is not necessary to keep the distance between the successive points at which the pressure is observed constant in order to obtain efiicient operation of a given order, if these distances are not the same the calculation of the proper impedance relationships becomes more diificult. The distances between the apertures should also be as large as possible in order to obtain high sensitivity. The limit on these distances is the effective phase difference between the points at which the pressure is observed when the distance between those points becomes an appreciable part of the wavelength. Therefore, insofar as both sensitivity and fidelity are concerned, it is generally most cificient to maintain the distances between the points at which the pressure is observed a constant. For example, in the case of the third order microphone described herein, each of the apertures which occupies an extreme position is spaced apart from its next adjacent aperture or apertures which occupy a mean position a distance equal to one-half the distance between the apertures occupying the mean positions.

Although only single embodiments of third order microphones in accordance with the present invention have been illustrated and described herein, it should be obvious to those persons skilled in the art that various changes and modifications are possible within the spirit of the invention. For example, the pressure pick-up points need not be disposed in a line but may be oriented in other fashion suitable to meet the requirements. However, random noise discrimination of a microphone having the pressure pick-up points in a line will usually be greater than that of a microphone having its pick-up points in any other orientation but which is otherwise identical.

What is claimed is:

1. In combination with a first order pressure gradient microphone, an adapter comprising a container, means providing separate acoustic chambers in said container, means to acoustically connect said chambers to said first order pressure gradient microphone, and a plurality of means disposed at ditferent pickup points in series spaced apart relation for transmitting sound energy from the ambient to said acoustic chambers, said pickup points being at least three in number and comprising two extreme points, said means disposed at said extreme points in said series being dimensioned to provide larger acoustic impedance than the means disposed intermediate said extreme points in said series.

2. In combination with a first order pressure gradient microphone, an adapter comprising a container having top and bottom members, means providing separate acoustic chambers in said container, said bottom member having an aperture leading to each of said chambers and means to acoustically connect said chambers to said first order pressure gradient microphone, said top member having at least four apertures disposed in series spaced apart relation for transmitting sound energy from the ambient to said acoustic chambers, said apertures at the extreme points in said series each being so dimensioned as to provide a larger acoustic impedance than said apertures at the main points in said series.

3. in combination with a first order pressure gradient microjionc, an adapter comprising a container having top and bottom members, means providing separate acoustic chambers in said container, means to acoustically connect said chambers to said first order pressure gradient microphone, and a plurality of means, said means being disposed in series spaced apart relation for transmitting sound energy from the ambient to said acoustic chambers, said means being at least four in number, said plurality of means at extreme points in said series each being dimensioned to provide an acoustic impedance which is substantially equal to twice the acoustic impedance of either of said means at mean points in said series, successive ones of said plurality of means being connected with alternate ones of said chambers.

4. In combination with a first order pressure gradient microphone, an adapter comprising a container, means providing separate acoustic chambers in said container, means to acoustically connect said chambers to said first order pressure gradient microphone, and a plurality of means, said means being disposed in series spaced apart relation for transmitting sound energy from the ambient to said acoustic chambers, said means being at least four in number, said plurality of means at extreme points in said series being dimensioned to provide an acoustic impedance which is substantially equal to twice the acoustic impedance of said means disposed intermediate said extreme points in said series, successive ones of said plurality of means being connected with alternate ones of said chambers.

5. In combination with a first order pressure gradient microphone, an adapter comprising a container having top and bottom members, and a wall describing a serpentine path to provide separate acoustic chambers in said container, means to acoustically connect said chambers to said first order pressure gradient microphone, and a plurality of means, said means having a corresponding plurality of apertures disposed inseries spaced apart relation for transmitting sound energy from the ambient to said acoustic chambers, said means being at least four in number, said plurality of apertures at extreme points in said series being dimensioned to provide an acoustic impedance which is substantially equal to twice the acoustic impedance of either of said apertures at mean points in said series, successive ones of said plurality of means being connected with alternate ones of said chambers.

6. In combination with a first order pressure gradient microphone, anadapter to convert said first order pressure gradient microphone to a third order pressure gradient microphone comprising a container, means providing separate geometrically similar acoustic chambers in said container, a pair of acoustically equal tubes to connect said chambers to said first order pressure gradient microphone, and a plurality of at least four means, said means being disposed in series spaced apart relation for transmitting sound energy from the ambient to said acoustic chambers, said plurality of means at extreme points in said series each being dimensioned to provide an acoustic impedance which is substantially equal to twice the acoustic impedance of either of said means at mean points in said series.

7. The invention as defined in claim 6 wherein each of said means at said mean points in said series include a pair of apertures.

8. In combination with a first order pressure gradient microphone, an adapter comprising a container having top and bottom members, a relatively flat Wall providing two separate substantially geometrically equal acoustic chambers in said container, said bottom member having a pair of apertures to acoustically connect said chambers to said first order pressure gradient microphone, and a plurality of at least four means, said means being disposed in series spaced apart relation for transmitting sound energy from the ambient to said acoustic chambers, said plurality of means at extreme points in said series each being dimensioned to provide an acoustic impedance which is substantially equal to twice the acoustic impedance of either of said means at mean points in said series, the first and third of said means being in substantial alignment with each other and leading to one of said acoustic chambers, the second and fourth of said means being in substantial alignment with each other and leading to the other of said acoustic chambers.

9. in combination with a first order pressure gradient microphone, an adapter comprising a cylindrical container having top and bottom members, a relatively straight flat wall to provide two separate geometrically similar acoustic chambers in said container, and means to acoustically connect said chambers to said firstorder pressure gradient microphone, said top member including a plurality of at least four apertures disposed in series spaced apart relation for transmitting sound energy from the ambient to said acoustic chambers, each of said plurality of apertures at extreme points in said series being dimensioned to provide an acoustic impedance which is substantially equal to twice the acoustic impedance of either of said apertures at means points in said series, said plurality of apertures further being in substantial alignment with said straight flat wall and being cut at angles of less than ninety degrees, the first and third of said apertures being cut to lead to one acoustic'chamber, the second and fourthof said apertures 'being cut to lead to the other 10 acoustic chamber.

References Cited in the file of this patent UNITED STATES PATENTS Olney et a1. Oct. 18, 1949 2,529,467 Wiggins Nov. 7, 1950 

